In Nominal terms, the S&P 500 has grown 289% over the past 20 years, compared to 160% for the S&P GDP (approximation based on 50/50 sales mix proportions; World GDP 182%, and US 137%):
Below statement added January 30, 2014: In full disclosure, market capitalization (not presently available) would be a better way to look at this, because there have been unprecedented stock repurchases in the S&P500, which are not reflected in my graph.
I have calculated what I feel is the appropriate factor range, 22% to 48.6%, to subtract from the "20 Year S&P 500 Valuation Growth vs. GDP Growth" chart above. Here is the chart used showing my calculation method- 20 Year S&P 500 Shares Buybacks Estimate As Percent of Market Capitalization: Blog Post
Therefore, the 20 Year S&P 500 Price change of 289.3%, when adjusted for stock repurchases, has a adjusted gain in the 267.3% to 240.7% range, with a mid-point of 254%. Compared to an estimated S&P 500 GDP change of 160%. Expressed another way, over 20 years the S&P 500 Price, adjusted for stock repurchases, changed 94% > S&P 500 GDP.